By peoplestaff225
Updated March 01, 2016 04:59 PM

How does Jennifer Lawrence, creator of the epically caloric spaghetti-chili-pizza sandwich, still manage to slip into all those gorgeous Dior dresses? Well, she owes some of it to her trainer and friend, Dalton Wong.

The London-based fitness guru first worked with the actress in 2010 when he trained her for X:Men: First Class and she continues to abide by his health tips today. “We spent 10-12 hours a day together for three months working out between filming on [the X-Men] set,” Lawrence wrote in the foreword of Wong’s new book, The Feelgood Plan (out March 8). “He changed my body for that movie and gave me the skills to change my life.”

The secret: A health and fitness plan that doesn’t deprive. “I could never live on a ‘diet.’ Dalton taught me how to eat, move and live a delicious but healthy life,” said Lawrence. “We remain good friends; when I’m in London I love to train in his gym, then afterwards we go out for burgers and fries with his family. It’s all about balance.”

And Lawrence inspired Wong in return. “I met Jen when she was 20, and although I had been doing this for a long time, she helped me figure out how to make a 20-year-old look amazing, but still do 20-year-old things,” Wong tells PEOPLE. “I would say, ‘okay, Jen, this is what we have to do. You have to exercise, you have to eat well, but you can go out on a Friday night, and yes, you can have a candy bar.'”

That meant teaching Lawrence – who will hit the big screen in X-Men:Apocalypse this spring – how to consume the appropriate amount of fats, proteins and carbs for her body. This included go-to snacks like turkey, hummus and vegetables, sourdough bread bites with a bit of avocado and even dark chocolate. “I was always liberal about her diet [when filming] but it was portion controlled,” says Wong, who made sure Lawrence had enough “fuel” in order to have energy to work.

This also meant exercising whenever and wherever possible. “In between scenes we would squeeze in 15 minutes of exercise – things we could do on the fly like squats and lunges,” says Wong. This led the trainer to incorporate the same principles in his book. “By taking 15 minutes to do something for your body that makes you feel good. You don’t just get an instant lift, you build a lasting change.”

So, Wong, who has also worked with Amanda Seyfried and Game of Thrones‘ Kit Harrington, packed the book with “realistic tips” related to everything from snacking (air-popped popcorn is a favorite) to consuming alcohol. For example, Wong says, “Champagne is one of the best [alcoholic drinks] to have because of portion control. It comes in a flute and is always in a controlled amount.” Plus, he adds “Champagne cuts through fatty foods, so it helps digestion.”

But, again, Wong says everything in moderation is key. “I tell my clients, you have to drink your carbs or eat them, so if you like wine you can’t have dessert.” Given, Lawrence’s love of cabernet, we’re guessing she’d not much of a dessert person.

–Michelle Ward Trainor